The present invention relates to the field of photovoltaic power generation. More specifically, the present invention relates to the field of utility-scale high-concentration photovoltaic power generation.
Silicon photovoltaic (PV) cells directly convert light into electricity. This provides a non-polluting renewable source of electrical energy. An electrical generating system utilizing PV cells is a PV system.
PV systems are generally made up of arrays of PV cells (PV arrays). PV arrays may be fixed, or may have one-axis or two-axis tracking. Fixed arrays are substantially immovable. At any given instant, maximum power output is obtained when the angle of solar incidence is perpendicular to the effective aperture (i.e., receptive surface area of the array). That is, a line between the sun and the array is perpendicular to the plane of the aperture. The average annual output of a fixed PV array is maximized when the array is aimed approximately at mean true solar noon. That is, the array would ideally be positioned so the plane of the aperture is substantially perpendicular to a line extending to the mean true solar noon position of the sun.
A PV array using one-axis tracking pivots around a single axis to better track the sun across the sky. Ideally, one-axis tracking causes the angle of solar incidence to be coincident with a plane perpendicular to both the plane of the aperture and the plane of rotation of the array. The average annual output would be maximized when the array is angled to track approximately through the mean true solar noon position of the sun.
A PV array using two-axis tracking pivots around two axes to best track the sun across the sky. Because two axes are used, the sun may be accurately tracked to provide a substantially perpendicular angle of solar incidence at all times from dawn to dusk every day of the year.
PV arrays may be non-concentrating or concentrating. In a non-concentrating array, sunlight falls directly upon the PV cells making up the array. The aperture is therefore the collective area of the PV cells.
Non-concentrating PV arrays produce power from sunlight with any reasonable positive angle of incidence. Non-concentrating arrays therefore lend themselves to fixed and one-axis tracking arrays, where the angle of solar incidence is not normally perpendicular to the plane of the aperture. This, of course, does not exclude a non-concentrating array from having two-axis tracking.
In a concentrating photovoltaic array, sunlight falling upon lenses or mirrors is focussed onto the PV cells making up the array. That is, the aperture is substantially equal to the area of the lenses or mirrors, and the sunlight is focussed onto the substantially smaller area of the PV cells.
Concentrating PV arrays require that the sunlight be focussed upon the PV cells. To do this, each PV cell is located substantially at the primary focal point of the associated lens or mirror. This means that the angle of solar incidence must be such that the sunlight is directed to the appropriate focal points. The effective plane of the aperture is therefore perpendicular to the solar incidence. A typical concentrating PV array uses two-axis tracking to maintain perpendicularity.
The higher the concentration of an array, the smaller the amount of PV cell area required for a given aperture. The maximum concentration is partly limited by the ability of the individual PV cells to handle and dispose of heat. High-concentrating photovoltaic (HCPV) arrays having concentrations between 200 and 300 are fully realizable.
A problem arises with HCPV arrays in that the high concentration necessitates a high tracking accuracy. This results in more precise (and more expensive) two-axis tracking mechanisms than are required for lower-concentration arrays.
Typical PV systems may be instrument-scale, small-scale, or utility-scale. An instrument-scale PV system typically uses a single non-concentrating fixed array to power an individual device. Typically, the PV array is proportionate to the device to be powered. That is, an array may contain one to dozens of PV cells and have a power output from a few milliwatts to several watts.
A small-scale PV system typically uses non-concentrating fixed or one-axis tracking arrays to fully or partially power a residence, commercial establishment, or an industrial or agricultural device (e.g., a remotely located pump). A small-scale PV system may be formed of one or more arrays, may contain from several hundreds to several thousands of PV cells, and have a power output in the range of one to twenty-five kilowatts.
A utility-scale PV system is a solar power-generation station, and serves essentially the same functions as fossil fuel or nuclear power-generation stations. Solar power generation has an advantage in that solar energy is a fully renewable, non-polluting resource. The sunlight is present each day weather permits.
The electricity produced by solar power-generation stations, however, currently has a considerably greater cost per megawatt than that produced by fossil fuel and nuclear power-generation stations. There exists a long-felt need for reductions in the per-megawatt cost of solar power generation to make utility-scale PV systems more feasible.
A utility-scale PV system may be used by a utility to produce power for the public power grid. The power thus produced may be used to augment the power already available on the public grid during the times of sunlight, thereby providing supplemental power when it is most needed. A utility-scale PV system may also be used to provide power where the public power grid is not available, e.g., a remote village.
A utility-scale PV system may be formed of large arrays or array clusters containing from thousands to millions of PV cells. Each large array or array cluster typically has a power output in excess of twenty kilowatts, with the system having a total power output of tens or hundreds of megawatts.
The arrays or array clusters of a utility-scale PV system may be independently coupled to the power grid. Therefore, while a PV system may have a number of substantially identical arrays, this is not a requirement. A given PV system may be a power-generation complex having a mixture of non-concentrating and/or concentrating fixed, one-axis tracking, and/or two-axis tracking arrays.
Utility-scale PV systems may have power outputs of tens or hundreds of megawatts. The per-unit-area fabrication costs of a PV array decrease as the array is increased in size. Also, the cost of PV cells (the silicon cost) is a significant factor in large arrays. The greater the ratio of aperture area to PV cell area, the lower the silicon cost of a given array. Concentrating arrays have fewer PV cells per unit area of aperture than non-concentrating arrays. At some point in the transition between small-scale and utility-scale systems, it becomes preferable to utilize large HCPV arrays over non-concentrating arrays. The use of HCPV arrays, however, requires more expensive two-axis tracking.
The arrays of a utility-scale PV system may be quite large. Such arrays are often too large to be transported by conventional means (e.g., by rail and/or truck). Such large arrays must be either fully or partially assembled in the field. This results in a significant increase in labor costs and in the time it takes to bring the array on line. When the arrays are HCPV arrays, the required tracking accuracy requires an increase in field alignment time. With conventional field assembly and alignment techniques, the result may be an untenable overall PV system cost.
Desirably, HCPV arrays are rigid. That is, an HCPV array desirably has all PV cells therein properly aligned at all times and in all attitudes. If an HCPV array has insufficient rigidity, then that array is subject to deflection. Deflection is the optical misalignment of one or more PV cells in an array due to bending of the array. A deflected PV cell has a different optical alignment than a non-deflected PV cell. An array may suffer deflection due to gravity (i.e., dead-load deflection) and wind (i.e., wind-load deflection). For arrays using lenses, the amount of dead-load deflection varies with the attitude of the array. That is, the dead-load deflection is most pronounced when the array is horizontal and substantially zero when the plane of the aperture is vertical.
Large HCPV arrays must be rigid enough to support their own weight while maintaining proper optical alignment of all PV cells therein. This typically results in arrays that have elaborate, cumbersome, and/or massive supporting structures. These complex supporting structures are themselves typically assembled on site, and therefore add to the cost increases.
Accordingly, it is an advantage of the present invention that an assembly of high-concentration photovoltaic modules is provided for use in a utility-scale power generation system.
It is another advantage of the present invention that the assembly may be fabricated and aligned in a factory.
It is another advantage of the present invention that the assembly may be readily transportable by truck from the factory to the PV system site.
It is another advantage of the present invention that the assembly is sufficiently rigid to support its own weight and maintain alignment during use.
It is another advantage of the present invention that the assembly contains a frame configured to support the assembly and couple the assembly to a supporting tracking structure of the PV system.
It is another advantage of the present invention that the assembly is one of a plurality of substantially identical assemblies configured to mount to one supporting tracking structure of a PV system.
The above and other advantages of the present invention are carried out in one form by a high-concentration photovoltaic assembly configured for use in a utility-scale power generation system. The assembly has a plurality of substantially parallel framing members substantially centered in a plane and configured to couple to a supporting tracking structure of the system. The assembly also has a plurality of substantially parallel longitudinal members substantially centered in the plane and coupled to the framing members proximate ends thereof. The assembly also has a plurality of photovoltaic modules coupled to adjacent ones of the longitudinal members upon a first side of the plane. The assembly also has a plurality of lenses coupled to adjacent ones of the longitudinal members upon a second side of the plane.
The above and other advantages of the present invention are carried out in one form by a high-concentration photovoltaic assembly configured for use in a utility-scale power generation system. The assembly has a frame substantially centered in a plane and configured to couple to a supporting tracking structure of the system. The assembly also has a plurality of substantially parallel longitudinal members coupled to the frame. The assembly also has two substantially parallel transverse members coupled to the longitudinal members substantially at ends thereof. The assembly also has a plurality of photovoltaic modules coupled to adjacent ones of the longitudinal members upon a first side of the plane. The assembly also has a plurality of lenses coupled to adjacent ones of the longitudinal members upon a second side of the plane.